This invention relates to a decoding device for use in decoding an encoded video signal, into which a video signal representative of successive pictures is compression encoded by a counterpart encoder. Such a decoding device is particularly useful in a television conference system.
In a compression encoding device of a television conference system, the video signal is a television signal. In this event, the successive pictures are represented by successive frames of the television signal.
The compression encoding device is, for example, a predictive encoding device and is effective to achieve data compression as is known in the art. The compression encoding device has an encoded data generating speed which is not constant but varies with time depending upon the characteristic of the video signal. The compression encoding device therefore comprises an encoder buffer memory in order to carry out speed conversion between the encoded data generating speed and an encoded data transmitting speed. The encoded data generating speed and the encoded data transmitting speed are equal to write-in speed and read-out speed, respectively, of the encoder buffer memory. The encoded data are transmitted from the compression encoding device to the decoding device through a transmission line. The decoding device for decoding the encoded data comprises a decoder buffer memory in order to carry out speed conversion between the encoded data transmitting speed and a data decoding speed, at which the encoded data are decoded in a decoder. The encoded data transmitting speed and the data decoding speed are equal to the write-in speed and the read-out speed, respectively, of the decoder buffer memory.
It is necessary to carry out the speed conversion by the encoder and the decoder buffer memories without partially losing or duplicating the data. In order to correctly decode the encoded data by the decoder, it is necessary to control the read-out speed of the decoder buffer memory. Each of the encoder and the decoder buffer memories has a delay time caused by difference between the write-in speed and the read-out speeds. The delay time of the encoder buffer memory is utilized for controlling the read-out speed of the decoder buffer memory. To describe more exactly, the delay time of the encoder buffer memory is detected at the compression encoding device. Information of the delay time is transmitted to the decoding device as delay time information through the transmission line. In the decoding device, read-out of the decoder buffer memory is controlled by using the delay time information so that a sum of the delay time in the encoder and the decoder buffer memories becomes constant.
The compression encoding device and the decoding device of the type described above are exemplified in U.S. Pat. No. 4,472,803 issued Sept. 18, 1984, to Yukihiko Iijima and assigned to NEC Corporation.
The delay time of the above-described decoding device is influenced by a maximum delay time in the compression encoding device because the read-out of the decoder buffer memory is controlled by the use of the delay time information. In other words, the delay time is determined for the decoder buffer memory by the maximum delay time of the encoder buffer memory.
In addition, the delay time of the encoder buffer memory increases in proportion to the amount of the encoded data per frame. On the other hand, the compression encoding device generates a large amount of encoded data on a scene change between the pictures. Therefore, the encoder buffer memory has a long delay time on the scene change. This means that the decoder buffer memory has also a long delay time even if the encoded video signal comprises the frames each of which has only a small amount of encoded data. Responsive to the read-out data of the decoder buffer memory, the decoding device decodes the encoded video signal into a decoded video signal.
The decoded video signal is used in reproducing images corresponding to the pictures represented by the video signal supplied to the compression encoding device. Regardless of the amount of encoded data, the images has a bad influence caused by the delay time of the decoder buffer memory.